1. Field of the Invention
This invention relates to a liquid crystal composition for liquid crystal display. More particularly, it relates to a liquid crystal composition used in a display element employing TN (twisted nematic) mode or STN (supertwisted nematic) mode, and to a liquid crystal display element using the composition.
2. Description of the Related Art
At present, display elements utilizing liquid crystals have been broadly utilized for watches, electronic calculators, etc. These liquid crystal display elements utilize the optical anisotropy and dielectric anisotropy of liquid crystal substances. Liquid crystal phases include a nematic liquid crystal phase, a smectic liquid crystal phase and a cholesteric liquid crystal phase. Among these, display elements utilizing nematic liquid crystal phase have been most broadly practically used. Further, as display modes applied to liquid crystal display, TN (twisted nematic) mode, STN (supertwisted nematic) mode, DS (dynamic scattering) mode, guest-host mode, ECB (electrically controlled birefringence) mode, etc. are mentioned. A number of liquid crystalline compounds including those in research stage until now, have appeared, but at present, there is no substance filled in display elements as a single liquid crystal substance and used. This is because, liquid crystal substances expected as display element material are preferred to exhibit liquid crystal phases within a temperature range as broad as possible, including room temperature at which the element is most frequently used, and should be sufficiently stable for use under environment factors and have physical properties sufficient for driving display elements, but those satisfying these conditions as a single substance have not yet been found.
Thus, at present, several kinds of liquid crystal substances and further, non-liquid-crystalline substances are blended to prepare compositions having required characteristics, and they have been subjected to practical use as display materials.
These liquid crystal compositions are required to be stable to moisture, light, heat, air, etc. usually present in the environment wherein they are used, and also stable to electric field and magnetic field, and further, it is required that mixed liquid crystal compounds are chemically stable to each other, under use environment.
Further, liquid crystal compositions should have suitable optical anisotropy, dielectric anisotropy, viscosity and a mesomorphic phase temperature range, depending upon display modes and use applications of elements.
As currently, most generally used display modes, TN (twisted nematic) mode utilizing the optical activity of liquid crystal cells having a twisted angle of 90.degree., and STN (supertwisted nematic) mode utilizing the birefringent property of liquid crystal cells set to a twisted angle of 180.degree. to 270.degree. are mentioned.
For the liquid crystal display elements utilizing these modes, it has become important to reduce the threshold voltage in the electrooptical characteristics and the temperature-dependency of the threshold voltage.
Namely, when the threshold voltage is reduced, low voltage drive and low consumed electric power become possible, and when the temperature-dependency of the threshold voltage is reduced, inferior displays such as poor contrast, cross-talk, etc. within a temperature range used are inhibited.
However, according to the current technique, reduction in the threshold voltage brings about inferior temperature-dependency and notable increase in the viscosity. Further, reduction in the temperature-dependency brings about increase in the threshold voltage and reduction in the steepness of the electrooptical characteristic curve. Thus, sufficiently satisfactory compositions have been very few.
As described above, since various physical and chemical properties are required for the liquid crystal compositions utilized for display elements, it is not easy to prepare satisfactory liquid crystal materials; hence it is the present status that liquid crystal compositions sufficiently satisfying these conditions have been still earnestly desired.